1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a program. More particularly, the invention relates to an image processing apparatus, an image processing method, and a program for detecting a change in an image using a limited storage capacity.
2. Description of the Related Art
There exist surveillance systems that take images of a predetermined target space using a surveillance camera in order to detect from the taken images intruders such as humans or animals into the target space. This type of surveillance system cannot conduct surveillance if its surveillance camera is diverted from its intended direction by an act of sabotage.
Some techniques have been proposed to bypass that weak point of the system. The techniques (e.g., proposed in Japanese Patent Laid-Open No. 2008-77517) involve calculating the difference in brightness value between the image currently taken by the surveillance camera and a previously stored reference image so as to determine that an act of sabotage has been committed if the calculated difference is above a predetermined value.
FIG. 1 is a schematic view explanatory of how images taken by a surveillance camera are updated. As shown in FIG. 1, when frames F1 through F6 are acquired sequentially, an image typically obtained every other frame is stored as the image of reference subject to sequential update. That is, this example involves the frames F1, F3 and F5 being updated sequentially as the reference image.
Where there is no act of sabotage, a chronologically previous background image and a chronologically subsequent background image remain the same and the difference in brightness value between the two images is small. By contrast, if the surveillance camera is diverted from its intended direction by an act of sabotage, the chronologically previous background image and the chronologically subsequent background image become different and the difference therebetween in brightness value becomes larger.
Thus when the frame F3 is obtained, the difference in brightness value is calculated between the frame F3 and the frame F1 that is the previous image serving as the reference image at this point in time. Likewise, when the frame F5 is acquired, the difference in brightness value is calculated between the frame F5 and the frame F3 that is the previous image serving as the reference image at this point. If the calculated difference turns out to be significantly large, then it is determined that an act of sabotage has been committed.